The usual prior-art carbon arc lamp and the electrode operating mechanism thereof have a structure as shown in FIG. 1. Upper carbon electrodes 1 and lower carbon electrodes 2 are mounted on an upper electrode holder 3 and a lower electrode holder 4, respectively. The electrodes and electrode holders within a glass housing 7 between an upper base 5 and a lower base 6. A drive section 8 is mounted on the upper base 5. The electrode holders 3 and 4 are coupled together by chains 9 extending over sprockets D and are vertically moved by the rotation of sprockets 10 by a motor 10a through bevel gears 10b. Slide members 11 and 11' move vertically along two upright supports 12 extending between and secured to upper and lower bases for guiding the electrode holders 3 and 4.
In this arrangement, the upright supports 12 and slide members 11 and 11' are subject to staining by the ash produced from the electrodes 1 and 2 during their discharge, and as ash gathers in the gap between the slide members 11 and 11' and the associated upright supports 12, the friction is increased. The accumulated ash is likely to become a solid mass and eventually cause the parts to stick, thus stopping the movement of the holders.
Accordingly, a large gap must be provided between each of the slide members and the associated upright support so that the sliding movement will not be prevented. Even with this arrangement, however, an undesired result cannot be completely prevented when the apparatus is used for a long period of time. In addition, because of the presence of the gap, the center position of the electrodes is subject to a shifting to a corresponding amount, causing a deviation from the alignment of opposed upper and lower electrodes and giving rise to abnormal discharge. Further, since chains are used to move the electrodes, blocking of the movement of the electrode holders is also likely to be caused by ash adhering to the chains and preventing of the normal functioning thereof.